Gas springs are well known and have been used in dies of presses for sheet metal stamping operations. Conventional gas springs include a cylinder, a piston disposed in the cylinder, a pressure chamber between the piston and cylinder, and various retainers and seals disposed in the cylinder to retain the piston and prevent leakage of pressurized gas from the pressure chamber. The pressurized gas provides a force on the piston to bias the piston toward an extended position and resists retraction of the piston from its extended position toward a compressed position.
As shown in prior art FIG. 14, typical gas springs are designed to produce a net force cycle C′ as a function of piston travel. The cycle C′ starts at a relatively high force, gradually increases over a compression stroke P′ and gradually decreases over an extension or return stroke R′, and ends substantially at the initial relatively high force. But as shown in prior art FIG. 15, other gas spring designs have been modified to produce a modified net force cycle C″. The cycle C″ starts at zero, then increases rapidly over an initial portion I″ of a compression stroke P″, thereafter follows conventional paths for the compression stroke P″ and a return stroke R″, but decreases rapidly over a final portion F″ of the return stroke similar to the initial portion I″ of the compression stroke P″, and ends at zero. According to accepted wisdom in the art for achieving such functionality, those of ordinary skill routinely resort to use of electromechanical controls, check valves in the pistons, and/or separate hydraulic and pressure chambers with one or more delay valves.